The brushless synchronous generator (BLSG) is widely used in aircraft and marine vessels for onboard power generation and it is also used as a shaft generator in energy efficient hybrid propulsion systems for marine vessels. The brushless excitation offers increased reliability and reduced maintenance requirements for the generator.
The exciter is a key component of a generator and the generator's output voltage is regulated by controlling the exciter's field current. The main function of the excitation system is to provide variable DC current to excite the main magnetic field in the rotor. Furthermore, it supports short time overload capability, controlling the terminal voltage with suitable accuracy, ensures stable operation and keeps the machine within permissible operating range.
The exciter machine and the rectifier are mounted on the same shaft as the main alternator. Excitation systems have a significant impact on the generator dynamic performance, availability, quality of generator's voltage and reactive power.
The present invention focuses on detecting a fault in a rotating diode of the rectifier.
Generally, there are two major fault conditions in the rotating rectifier of a brushless excitation system, namely                i. Open circuit failure        ii. Short circuit failure        
The failures could be single-diode failure or multiple-diodes failure, and each could be either short circuit or open circuit fault.
Having removed the need for brushes, commutator and slip rings, direct fault detection in the rotating rectifier in AC brushless exciter becomes very challenging. Nevertheless it is important to detect and react suitably to rotating diode failures.
The output capacity of the exciter is reduced during diode open circuit failure. In this case, the field current increases stress on other devices and also reduce the transient capability of the machine. For this type of failure, the generator is not at any immediate risk of terminal serious damage. Therefore it can continue to operate for a limited time. However, this increases stresses on the other diodes and could lead to further diode failures. Moreover, the voltage regulator could be damaged due to increased excitation.
A diode short circuit is the most frequent failure condition. During diode short circuit, the output of the exciter is severely affected and the main alternator is unable to provide rated voltage without overloading the exciter. Furthermore, a shorted diode is one of the most severe fault conditions, which requires a very large increase in exciter's field current to maintain the alternator voltage. If this fault condition persists, the exciter and/or the voltage regulator could be damaged. In most short circuit diode cases, the generator is forced to shutdown (tripped) to prevent permanent damage to the overall system.
Hence, there is a need to develop suitable fast and accurate methods for detecting such diode failures.
The prior art discloses some techniques for detecting diode failures.
For example, US20110216449 discloses a method and apparatus for fault detection of series diodes in rectifiers, wherein the voltage across one or both of the individual diodes, and/or the voltage across the pair of diodes are measured to determine a voltage ratio. The voltage ratio is then analysed to determine if a diode fault is present. By employing a voltage ratio rather than a fixed threshold, the fault detection can be used at all possible operating voltages, corresponding to machine operating conditions ranging from no load to full load.
The schematic diagram of a synchronous machine with a plurality of diode detection modules and series module connected to a fault detection module are shown in FIG. 1 and FIG. 2 of US20110216449. The transmission of the signal from the rotor to the stator is via a telemetry transmitter module 80 to a telemetry receiver module 90. The transmitter module 80 can sample, digitise and transmit data of the rotating elements, including that of the diode fault detection modules 20A-20F using wireless techniques.
However, this proposed solution uses 12 separate voltage sensors arranged across the pairs of series diodes. These voltage sensors measure the diode voltage and determine the ratio between two of these voltages, which is then analyzed to determine the fault. However this method is complex due to presence of larger number of sensors and due to the presence of series connected diodes.
U.S. Pat. No. 5,453,901 discloses a diode short circuit detection and protection circuit for excitation system of brushless synchronous machine. An RC (resistor-capacitor) circuit is used for detecting AC voltages in an exciter field winding for the purpose of directly operating a circuit breaker to remove excitation to the field windings and supply, temporarily shorting the AC current through the exciter field winding.
Failure of a single diode short circuit can result in hazardous conditions due to generation of high voltages in the exciter field, high current in the exciter armature, and loss of excitation and control.
Thus, ideally, a rapid response of a detection and protection system is required in order to prevent subsequent damage to the exciter windings and the voltage regulator.
In detail, a shorted diode fault would generate a large AC voltage in the field windings, and the protection circuit would respond to the generated AC voltage by temporarily shorting a RC circuit to protect the field winding. The temporary shutdown of the field windings removes all excitation from the generator field, as well as the field supply, which eliminates further damage to the excitation system.
Furthermore, the rotating rectifier adapts a non-standard bridge configuration of parallel-fused diodes redundant topology, which under diode shorted circuit, excessive current will burn the fuse, leaving the redundant branch taking all the rectification purpose, without the need of shutting down the entire generator.
However, in the configuration of U.S. Pat. No. 5,453,901, under normally balanced operating conditions, i.e. in the absence of rotating diode faults, the exciter armature winding shows highly linear but balanced behaviours. Thus, any diode fault emulated will upset the exciter armature winding balanced behaviour immediately. And, the exciter armature winding exhibits highly imbalanced effects due to the loss of rectification at the main field winding. Indeed, the reflected harmonics due to armature reaction effect is seen directly at the exciter armature winding, while exciter field winding will require more time to generate the significant fault signature due to reflected harmonics in the exciter armature winding.
Thus, there is a need for an improvement in the detection of a diode fault in a rotating diode rectifier circuit, e.g. in a brushless exciter, such as that used in a brushless synchronous generator, so that faults can be detected quickly, and preferably with minimal additional burden on, or modification of, the existing circuitry.